It is known that the numerical control (“NC”) of a machine tool, for example a machining center, enables to carry out automatic machining operations of a mechanical piece by controlling mutual displacements between the tool and the piece with very high accuracy. The control is carried out by means of suitable instructions, on the basis of previously acquired numerical data.
It is also known to use sensor devices such as checking or measuring units to detect the dimensions or the position of the mechanical piece being machined and/or of the utilised tool or other component parts of the machine, and to provide associated signals to the numerical control. The numerical control employs new data obtained on the basis of said signals in order to enhance the controls and the piece machining.
Even the intervention of the sensor devices is controlled by the NC in an automatic or manual way by means of suitable instructions. For instance, in the numerical control there is available a measuring instruction using a digital signal (trigger) produced by the external sensor device. This measuring instruction generally requires a displacement between component parts of the machine and the sensor, and “photographs” the positions of the machine axes (detected by suitable devices) under the control of the sensor device. The displacement ends if the sensor signal is received or, in case of non-reception of the latter, if the movable component parts reach a borderline safety position.
The sensor device can be for example a touch probe or an optoelectronic device (“laser probe”) including a light beam transmitter and a light beam receiver and means for signalling the interruption of said beam. In both cases, there is included an interface unit for sending the trigger signals to the NC through a measuring channel, which is generally defined by a connection cable.
Sometimes, a second cable couples the sensor device or the associated interface unit to the NC for sending a signal indicative of the correct operation of the sensor device. For instance, in the event a “laser probe” is employed, the signal indicates that the light beam correctly reaches the receiver.
A problem of the known apparatus is related to the possibility of checking the correct operation of the sensor device in a very reliable way in order to avoid wrong signalling and dangerous uncontrolled displacements of the component parts of the machine tool.
In fact, a second physical input in the NC for the previously mentioned second connection cable is not always available. Moreover, even when the second connection is available, the possibility that a failure, for example in the interface or in the second connection, causes a wrong positive signalling is not negligible. Finally, the checking is in any case partial, since the condition of the measuring channel, which could be interrupted or in short circuit with others conductors, is not taken into consideration.